1. Field of the Invention
The present invention concerns a magnetic resonance system of the type having a base body and a patient bed, wherein the base body contains a magnet system that generates magnetic fields in an excitation region to excite nuclear spins in a patient in the excitation region, and wherein the patient bed is movable in a travel direction relative to the base body.
2. Description of the Prior Art
Magnetic resonance systems of the above type are generally known.
Conventionally the acquisition of the magnetic resonance signals emitted by the examination subject ensues either with a whole-body antenna or with local coils. If the magnetic resonance signal is acquired by a whole-body antenna, the magnetic resonance signal can be acquired from the entire excitation region. The acquisition is possible only with relatively low signal-to-noise ratio (SNR). Therefore in many cases local coils are also used in magnetic resonance systems, often multiple local coils. The local coils are arranged in proximity to the examination subject (normally a person) and can therefore acquire with good SNR, even if only from a small part of the excitation region. Moreover, due to the spatial resonance by the arrangement of the local coils the spatial coding with gradient fields can as such be extended. The required measurement time for an acquisition can thus be reduced.
In order to be able to utilize the advantages of local coils for imaging of the entire body of a person from head to foot, a large number of local coils is required that are placed on the patient in a number of planes arranged along the travel direction of the patient bed. These planes are often designated as levels.
The excitation region typically exhibits a length of approximately 40 to 60 cm as viewed in the patient bed travel direction. Only a portion of the body of the person can therefore ever be imaged, namely the portion that is located in the excitation region. For this reason the patient bed with the patient located thereupon must be moved bit by bit through the excitation region. The local coils located in the excitation region must respectively be activated and be connected with an evaluation device for evaluation of magnetic resonance signals. The other local coils can be deactivated. It is thereby possible to manage with a relatively low number of reception channels even though many local coils are present.
In conventional magnetic resonance systems the aforementioned type of examination is conducted by connecting all local coils, via a corresponding number of plugs at the patient bed and via a movable cable harness, to an evaluation device that is arranged at the base body i.e., the stationary part of the MR apparatus. Due to the attenuation of the long, thin cables that are used, pre-amplifiers must be arranged in the local coils. Each local coil must furthermore have a detuning circuit in order to be able to be deactivated upon non-usage and given transmission. Complicated common mode chokes (known as sheath wave barriers) must also be inserted into the long cable harness in order to be able to limit induced voltages that occur in the transmission mode.
For the back region of the patient it is possible to arrange the acquisition array in a fixed manner under the movable patient bed in the excitation region. In this case the number of these local coils must only be sufficient for the excitation region. If the examination concerns the back region of the patient, it is therefore possible to save many local coils, their cabling and the channel selection. The distance from the examination subject is increased only by the relatively slight thickness of the patient bed itself, which is most cases quite tolerable.
By contrast, this procedure cannot be used or can just barely be realized on the top side of the examination subject since the thicknesses of the patient and the various body regions of the patient are very different. Primarily for thin patients or, for example, at the head or legs, an acquisition coil array permanently installed in the excitation region would be far removed from the body surface, such that the advantage of the local coils (namely a high spatial resolution and a good SNR) would be lost.
A plug connection for local coils that operates without contact (namely via inductive coupling) is known from DE 101 30 615 C2. This teaching already represents an advance since a galvanic contact between the local coil and the evaluation device is no longer required for coupling of a local coil to the evaluation device. However, as before the requirement of an active plugging of the connection by an operating personnel exists. The local coil must also be manually connected to the evaluation device or disconnected therefrom.
From DE 35 00 456 C2 it is known to couple a local coil with the whole-body antenna. Here a contactless coupling is in fact realized. However, this coupling is only possible for a single coil, and even then only given a suitable orientation of the local coil. The teaching of DE 35 00 456 C2 thus cannot be extended to a number of local coils. Here as well the local coil must be actively connected to the whole-body antenna or disconnected therefrom.
From EP 0 437 049 A2 it is known to directly, inductively couple one local coil to another coil that is arranged in the immediate proximity of the local coil. Again, the local coil must be actively connected to the evaluation device or disconnected from it.
A magnetic resonance system with a local coil is known from DE 197 51 017 A1, wherein the local coil is inductively coupled with a decoupling coil which is connected with the evaluation device via electrical conductors.
A magnetic resonance system with a local coil is known from U.S. Pat. No. 5,243,289, wherein the local coil is connected with coupling elements which are inductively coupled with an inductor, and the inductor is connected with the evaluation device. The degree of the inductive coupling can be adjusted by variation of the relative position (distance and/or overlap) of the inductor and the coupling elements.
A magnetic resonance system with a local coil is known from DE 39 35 082 D1, wherein the local coil is connected to a plug connection that is arranged at the patient bed.